Genetics of resistance to sterility mosaic disease in pigeonpea

LIST OF ILLUSTRATIONS

1. Distribution of pigeonpea in India (1987-90)

2. Prevalence of pigeonpea sterility mosaic disease in different states of India (1975-80)

LIST OF PLATES

1 Sterility mosaic disease of pigeonpea

A Severe mosaic symptoms

В Vector - Acena cajam C

C Infected and sterile vs Healthy and flowenng

2 a) Screening against sterility mosaic disease of pigeonpea using infector-hedge technique

A Field screening

В Pot screening

b) Types of symptoms noticed against pigeonpea sterility mosaic disease A Resistant

В Ring spot

C Mild mosaic

D Severe mosaic

3 Hybridization and setting m pigeonpea

A Crossing of plants raised in pots

В Emasculated bud

C Pollinated and tagged bud

D Setting in bee-proof nylon cages

4 a) Off-season advancement of generations 32

A Using black-out facility

В Backcrossing with parents maintained as ratoon

b) Screening in mite-proof net house

c) Leaf-stapling technique

A Using single diseased leaflet

В Using two small leaflets

LIST OF TABLES

1. Reaction of pigeonpea differential genotypes to variants of sterility mosaic pathogen in India (1987-90)

2. Sterility mosaic reaction of five pigeonpea lines at Nepalganj. Nepal and ICRISAT Centre. India

3. Range of mid-parent, better-parent and standard heterosis for yield and yield component characters In pigeonpea

4. Gene action for yield and yield component characters in pigeonpea

5. Broad sense heritabillty (%) for yield and yield component characters in pigeonpea

6. Parents selected for studies on inheritance of resistance to different isolates of the sterility mosaic pathogen

7. Salient features of the selected pigeonpea lines

8. Pigeonpea lines resistant/tolerant to isolate 1 of sterility mosaic pathogen

9. Pigeonpea lines resistant/toierant to isolate 2 of sterility mosaic pathogen

10. Reaction of parents, and F, hybrids for isolate 1 of pigeonpea sterility mosaic pathogen

11. Reaction of segregating generations of resistant x susceptible and tolerant x susceptible crosses to isolate 1 of pigeonpea sterility mosaic pathogen

12.

Reaction of parents for isolate 2 of pigeonpea sterility mosaic pathogen

13. Reaction of F, hybrids of resistant x susceptible crosses to isolate 2 of pigeonpea sterility mosaic pathogen

14. Reaction of F2 generation of resistant x suscepttole crosses to isolate 2 of pigeonpea sterility mosaic pathogen

15. Reaction of F, hybrids of resistant x resistant and susceptible x susceptible crosses to isolate 2 of pigeonpea sterility mosaic pathogen

16. Reaction of Fj generation of resistant x resistant and suscepttole x susceptible crosses to isolate 2 of pigeonpea sterility mosaic pathogen

17. Reaction of parents, F, and F3 generations of resistant x suscepttole crosses for isolate 3 of pigeonpea sterility mosaic pathogen

18. Analysis of vanance (mean squares) for yield and yield component characters of parents and crosses in pigeonpea

19. Mean performance of parents for yield and yield component characters of pigeonpea

20. Mean performance of hybnds for yield and yield component characters of pigeonpea

21. Per cent heterosis over mid-parent (MP) and better parent (BP) of few yield component characters in pigeonpea

22. Per cent heterosis over mid-parent (MP) and better parent (BP) of yield and few yield component characters in pigeonpea

23. Analysis of variance (mean squares) for combining ability estimates of variance components and their ratios for yield and yield component characters in pigeonpea

24. Estimates of general combining ability effects of various parents for yield and yield component characters in pigeonpea

25. Estimates of specific combining ability for yield and yield components in pigeonpea crosses

26. Range mean coefficient of vanability heritability and genetic advance for yield and yield component traits in pigeonpea

27. Character associations for yield and yield component characters in pigeonpea

28. Direct and indirect effects of yield component characters on gram yield in pigeonpea

29. Comparative study of inheritance of resistance to three isolates of pigeonpea

30. Details of high yielding pigeonpea crosses for seed yield per plant

31. Pigeonpea crosses with substantial useful heterosis for yield and yield component characters

32. Details of promising hybrids identified for different maturity groups

33. Characterization of parents for per se performance and general combining ability effects

ACKNOWLEDGEMENTS

It gives me pleasure to express my gratitude to Dr. M. Shiva Shanth Reddy. Professor and University Head, Department of Genetics and Plant Breeding, College of Agriculture, Rajendranagar and Chairman of the Advisory Committee, for his valuable guidance, constant encouragement and constructive criticism during the entire course of this investigation

I am also grateful to my Co-chairman of the advisory committee, Dr. M.V. Reddy, Senior Scientist Legumes Pathology, Crop Protection Division, ICRISAT Asia Center, Patancheru, Andhra Pradesh for his co-operation, constant supervision and helpful suggestions in overcoming hurdles dunng the course of investigation

I am also extremely obliged to Dr. K.C. Jain, Senior Scientist, Genetic Enhancement Division, ICRISAT Asia Center, Patancheru. Andhra Pradesh, for his guidance, constant encouragement and moral support, during my most trying times and help in bnnging out, the best of my ability, in this dissertation I am also thankful to Dr. G. Raghunatham, Associate Professor, Department of Genetics and Plant Breeding. College of Agriculture, Rajendranagar, Hyderabad and Dr. D. Raja Ram Reddy. Associate Professor, Department of Plant Pathology, College of Agnculture, Rajendranagar, Hyderabad for their valuable assistance dunng the course of investigations

My sincere thanks are also due to the staff of Department of Genetics and Plant Breeding, College of Agnculture, Rajendranagar, Hyderabad and various units of ICRISAT Asia Center, Patancheru Hyderabad, friends, colleagues, well wishers, parents and others, whose contribution behind the scene, has enabled the successful completion of my investigations

Finally, I am obliged to A P State Government, Hyderabad, Council of Scientific and Industrial Research (CSIR), New Delhi and, Training and Fellowships Program, ICRISAT Asia Center, Patancheru Andhra Pradesh, for providing financial assistance and research facilities

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DECLARATION

I, T. Srinlvas. hereby declare that the thesis entitled 'GENETICS OF RESISTANCE TO STERILITY MOSAIC DISEASE IN PIGEONPEA (Cajanus cajan (L) Millsp.)" is a result of the original research work done by me. It is further declared that the thesis or any part thereof has not been published earlier in any manner.

(T. Srinlvas)

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ABSTRACT

Investigations on genetics of resistance to stenlity mosaic (SM) disease in pigeonpea were carried out (1993-96) at ICRISAT Asia Center, Patancheru, India, to determine the inheritance of resistance for three different isolates of pigeonpea sterility mosaic pathogen and to study the combining ability of few resistant and tolerant lines

In this direction, available lines (resistanVtolerant) were screened against two isolates of the SM pathogen Breakdown of resistance was noticed in several lines, against isolate 2 of the pathogen However, few lines resistant and tolerant to both the isolates were identified

Inheritance of resistance and tolerance, in few of these lines was investigated Screening was carried out In pots, using leaf-stapling technique for isolates 1 and 3 and infector-hedge, for isolate 2 Observations in F, and segregating generations indicated the recessive nature of resistance and role of two independent non-allelic genes for isolates 1 and 3 Resistance against these isolates appeared to be dependent on the presence of recessive alleles, at least at one of the loci However, against isolate 2 resistance was observed recessive in some crosses and dominant in other crosses Further disease reaction for isolate 2. appeared to be governed by two independent non-allelic genes with at least three multiple alleles, at one of the loci

Combining ability studies of the resistant, tolerant and susceptible lines included in the inheritance studies, were earned out with line x tester mating design, involving two male stenles and eleven pollen parents The analysis of vanance revealed significant differences for parents, hybrids, parents vs hybrids and males, for all characters studied Pre-ponderance of non-additive gene action was recorded for yield and all yield component characters studied

ICP MS288 female was found to be a good combiner for early maturity, dwarf and compact growth habit while ICP MS3783 tolerant to isolate 1 of pigeonpea sterility mosaic pathogen and wilt disease was better combiner for seed yield, pods per plant, test weight, pnmary and secondary branches Among the males, LRG 30 recorded high general combining ability, for seed yield and majority of yield components The sterility mosaic resistant parents were however, poor combiners for yield and majority of the component characters

The expression of heterosis was most evident for yield per plant, pods per plant and number of secondary branches It was maximum in mid-late x medium crosses, followed by early x medium crosses Significant and desirable sea effects were also recorded in several hybnds, for vanous traits studied Crosses with high sea effects for yield, were further found associated with high and desirable sea effects for most component characters The studies on variability, hentability, genetic advance, character associations and path analysis had also indicated the need for selection based on component characters such as pods per plant and plant height

Four promising hybrids (ICP MS288 X ICP 7349, ICP MS3783 X BDN 1 ICP MS3783 X LRG 30 ICP MS3783 X ICP 8863) were identified, in the present study, based on their per se performance heterosis and sea effects Of these, ICP MS3783 X BDN 1. ICP MS3783 X LRG 30 and ICP MS3783 X ICP 8863 crosses, Involved parents with high gca effects, indicating the role of fixable additive x additive gene Interactions These may hence be advanced through conventional breeding procedures coupled with screening and selection for resistance, pods per plant and plant height towards development of high yielding disease resistant cultivars

LIST OF ABBREVIATIONS

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CHAPTER I INTRODUCTION

Pigeonpea (Cajanus cajan (L.) Millspaugh) is one of the major pulse crops of the tropics and subtropics. It is widely grown in the Indian subcontinent, which accounts for almost 90 per cent of the world’s crop (Nene and Shlela, 1990). In India, It is grown in almost all states, but the major concentration is In the state of Uttar Pradesh in northern India, eastern parts of Gujarat and Maharashtra and north­eastern parts of Karnataka in Western India, and western parts of Madhya Pradesh in central India (Fig.1). It is widely used as a pulse, green vegetable, fodder, and for a variety of other purposes (Nene and Shieta. 1990). The seed protein content of pigeonpea (21%) compares well with that of other important grain legumes. The average yields of the crop are however, very low (750 Kg ha'1). High sensitivity of the crop to the attack of insect-pests and diseases appears to be the main reason for such disappointingly low yields.

The crop is attacked by more than 100 pathogens (Nene et al., 1996) including fungi, bacteria, viruses, mycoplasma like organisms and nematodes. However, only a few of them cause economic losses (Kannaiyan et al., 1984). The diseases of considerable economic importance at present are sterility mosaic (SM), Fusarium wilt, Phytophthora blight (PB), Macrophomina root rot and stem canker, and Altemaria blight in the Indian subcontinent.

Sterility mosaic is the most important disease of pigeonpea in India and at times can cause yield losses upto 95 per cent (Reddy and Nene, 1981). An annual loss of 205,000 tonnes of grains, valued at Rs. 676.5 millions is estimated due to the disease (Kannaiyan et al., 1984). The disease was first reported from Pusa in Bihar in India (Mitra, 1931). However, of late, It has posed a serious threat to the successful cultivation of pigeonpea in several parts of India (Lai et al., 1981). It is present in all major pigeonpea producing states and is a serious problem in north eastern (Bihar and Uttar Pradesh), and southern (Tamil Nadu) states (Kannaiyan et al., 1984). Prevalence of the disease in various states of India is presented in Fig. 2. No satisfactory cultural control has been found so far, to protect the crop from this disease (Singh

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et al, 1983) Further, chemical methods of control, while effective are not considered economical (Nene et al, 1989) Therefore, breeding of resistant varieties, recognized as the most effective and economic method of reducing crop losses (Stakman and Harrar, 1957) has received high priority for the disease

Development of resistant pigeonpea cultivais against the disease was first initiated by Alam (1931 ) Systematic resistance breeding was later initiated at ICRISAT, Patancheru, India in 1975, and several resistant and tolerant source(s) for the disease were identified (Nene et al, 1981) The genetics of resistance for the disease was also worked out (Singh et al, 1983 , Sharma et al, 1984) However, the task of developing resistant varieties has been complicated in view of the reported genetic plasticity of the pathogen The presence of strains of $M pathogen of varying virulence was reported by Nene et al (1989), based on the results of multi-location pigeonpea tnals Lines resistant at International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Patancheru broke down, when grown at other locations within India A comprehensive study of the phenomena, by Reddy et al (1993) over a period of four consecutive years, using a set of seven differentials at nine different locations in India, revealed the occurrence of five different vanants of the stenlity mosaic pathogen of pigeonpea in India

The foregoing knowledge, on the dynamic nature of stenlity mosaic pathogen has warranted the identification and use of strain-specific sources of resistance in crop improvement programs Further, it has also necessitated studies on genetics of strain-specific resistance to aid resistance breeding programs Pigeonpea improvement programs aimed at evolving high yielding disease resistant vaneties may be earned out effectively if information is available on combining ability of the recipient and donor parents This is more so, In the case of stenlity mosaic disease, since most of the donors are poor yielders Application of line x tester mating design (Kempthome 1957) was suggested for pigeonpea (Green ef al, 1979) to obtain information on the combining ability of the lines involved, for traits of economic importance towards identification of potential parents and cross combinations Further, owing to the existence of male sterility (Reddy et al, 1979 , Wallis et al, 1981) and a considerable degree of natural out-crossing (Green et al, 1979 , Onirn, 1981), evaluation of a large number of lines, for their combining ability has become possible, adopting the line x tester mating design

An understanding of the nature and magnitude of existing variability for important yield contributing characters is also necessary for a successful breeding program (Singh et al, 1995) Selection for yield per se generally remains unsuccessful in achieving desirable results, because yield is dependent on its various component characters Therefore, knowledge of association and cause and effect relationship of yield component traits with yield would help in formulating effective selection schemes (Talwar and Joshi, 1983)

The present Investigation was hence undertaken with the following objectives

1 To determine the genetics of strain-specific resistance for stenlity mosaic pathogen of pigeonpea
2 To study the combining ability of few resistant and tolerant cultivars
3 To study the nature and extent of genetic variation and character associations for yield and other economic traits
4 To suggest a suitable breeding strategy for exploitation of the material towards development of high yielding resistant vaneties

CHAPTER II REVIEW OF LITERATURE

A brief review of the relevant literature for the present investigation is presented hereunder.

2.1 GEOGRAPHICAL DISTRIBUTION, SYMPTOMS AND ECONOMIC IMPORTANCE

The disease was first reported from Pusa in Bihar, India (Mitra, 1931). Subsequently, it was reported from Tamil Nadu, Maharashtra, Gujarat, Punjab and Uttar Pradesh states (Capoor, 1952). Now, the disease is known to occur in all major pigeonpea producing states of India and is a serious problem in north-eastern (Bihar and Uttar Pradesh) and southern (Tamil Nadu) states (Kannaiyan et al., 1984). Its incidence in farmer’s field is reported to vary between 0 and 100 per cent. The disease has also been reported from Bangladesh, Nepal and Thailand (Nane er al., 1989), Myanmar (Su, 1931) and Sri Lanka (Newton and Peiris, 1953).

The disease, characterized by proliferation, mosaic symptoms, stunting, cessation of reproductive growth and a reduction in the size of the leaflets (Plate 1A), is transmitted by an eriophyid mite vector, Aceria cajani Channabasavanna (Plate 1B). It results in sterility of the plant (Plate 1C), which directly affects the yield. Alam (1933) reported a negative correlation between the degree of sterility and yield.

Reddy and Nene (1980) made a systematic study on the estimation of yield loss in pigeonpea due to sterility mosaic. They found 95 per cent yield loss when the plants got infected at seedling stage. The susceptibility of plants decreased with age. However, infection upto 45 days after planting mostly resulted in complete sterility. The number of secondary and tertiary branches increased along with prolonged duration of crop when the plants were infected at early stages. The yield toss also varied with the genotype. Some genotypes, such as ICP 2376, that exhibited ring spot symptoms did not show any sterility and suffered no obvious yield loss while, genotypes such as NP(WR) 15, that developed mild mosaic symptoms, were partially sterile and their yield toss was less (19-64%). The disease incidence was usually

Plate 1 Sterility mosaic disease of pigeonpea

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higher in ratooned and perennial pigeonpea Losses from sterility mosaic in India are about double those from wilt (Fusarium udum) the second most important disease amounting to 205 000 tons annually valued at Rs 676 5 millions (Kannaiyan et al 1984)

2.2 PATHOGENIC VARIABILITY

The pathogen causing the disease may be a virus (Capoor 1952) but its exact identity is yet to be established (Reddy ef al 1990) Possibility of more than one strain of pigeonpea sterility mosaic pathogen has long been suspected on the basis of differential disease reactions observed on some host genotypes in the multi location trials (Nene et al 1989) Lines resistant at ICRISAT Patancheru India had broken down when grown at other locations within India Virulence of SM isolates from Bangalore Dholi Vamban and Varanasi was higher compared to those at Badnapur Hyderabad Pantnagar Kanpur Ludhiana and Faizabad

A virulent form of the Patancheru strain of SM pathogen was noticed in the wilt and sterility mosaic screening nursery at ICRISAT Center Patancheru India during 1990/1991 rainy season (Reddy et al 1991) It was identified based on the altered reaction of ICP 2376 Ring spot symptoms that had been consistently observed on the line in the screening nurseries between 1975 and 1990 turned to severe mosaic in 1991 Resistance also broke down in tew other pigeonpea cultivars against the new isolate

A comprehensive study of variability in the SM pathogen of pigeonpea was taken up between 1987 and 1990 to clarify the differential reactions and breakdown of resistance noticed in multi location tnals (Reddy et al 1993) The study involved sixteen pigeonpea genotypes tested for their reaction to isolates of the sterility mosaic pathogen from nine disease endemic locations in India Differential reaction of seven genotypes (Table 1) noticed in 51 field and pot tests was used to categorize the nine isolates into five distinct groups The isolate from Gwalior was designated as variant 1 Badnapur and Patancheru isolates as variant 2 Coimbatore Kumargunj and Pudukottai isolates as variant 3 Bangalore and Dholi isolates as variant 4 and Kanpur isolate as variant 5 Thus five different variants J the sterility mosaic pathogen were reported to occur in India Further a comparison of the strains of sterility mosaic pathogen of

pigeonpea, in Nepal and India (Cbaurasia, 1993), using a set of five differentials (Table 2), revealed differences in the strains of SM. prevalent at ICRISAT Center, India and Nepalganj, Nepal.

Table 1 Reaction of pigeonpea differential genotypes to variants of sterility mosaic pathogen in India

(1987-1990)

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Table 2 Sterility mosaic reaction of five pigeonpea lines at Nepalganj, Nepal and ICRISAT Center, India

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2.3 IDENTIFICATION OF SOURCES OF RESISTANCE

Alam (1933), was the first to make observations on resistance to sterility mosaic. He reported Sabour 2E (Arhar) and some other Sabour types of pigeonpea to be resistant Kandaswamy and Ramaknshnan (1960) reported all 176 pigeonpea varieties grown at Coimbatore as susceptible to the disease Seth (1965) suggested Atylosia IW 1431 as a promising material for incorporation of resistance into pigeonpea cultivars

Ramaknshnan and Kandaswamy (1972) identified NP(WR) 15 P1100 P 1289 P 1778 P2621 A and P 4834 lines as tolerant However they were unable to identify good sources of resistance out of 4514 collections tested Further Janarthan et al (1972) tested 18 varieties and found them susceptible to the disease Similarly Subramaman et al (1973) found all 549 pigeonpea lines tested as susceptible to stenlrty mosaic Further as many as 234 germplasm lines were evaluated for their reaction to sterility mosaic dunng 1973 74 at Ludhiana (Singh et al 1975) Out of these L 3 and P 47895 were found resistant while 16 others were tolerant Rath (1977) reported hr es P 4785 and L 26 as resistant but L 3 as susceptible

Systematic efforts were initiated at ICRISAT Center in 1975 (Nene and Reddy 1976b) About three thousand accessions were screened for resistance to sterility mosaic (№ne and Reddy 1976a b) Five pigeonpea lines ICP 2376 3783 6497 7035 7119 were identified as immune Rathi (1977) found ICRISAT lines ICP 3783 5444 6497 7035 7119 and Pantnagar lines Pant В 76 Pant В 77 and E 41 free from disease Rathi (1980) screened germplasm lines in sterility mosaic sick plot and identified 25 resistant lines

Nene et al (1980) reviewed the work on resistance of pigeonpea to sterility mosaic carried out at ICRISAT A total of 7555 germplasm lines and 10 Atylosia species were screened for resistance to stenlity mosaic at ICRISAT Center during 1975 80 Of these 66 resistant lines were identified directly from germplasm 433 resistant lines were developed through single plant selections and 54 lines were identified tolerant One Atylosia species (A volubilis) was found to be resistant Thirty five lines were found to be resistant at more than one location Venkateswarlu et al (1980) identified 28 pigeonpea lines free from sterility mosaic out of 90 lines tested Gupta et al (1981) identified 15 sterility mosaic resistant earfy maturing lines (upto 140 days) with higher yield Zote and Dandanaik (1986) identified six resistant and two tolerant (ring spot) lines among 22 lines screened against stenlity mosa - during 1981 84 at Badnapur india

In field screening trials (Kusum Dwivedi and Shukla 1986) with 20 cultivars exposed to natural infection of sterility mosaic three lines were observed tolerant with localized nng spots on the leaves Four lines were found moderately susceptible while the rest were susceptible Field screening with infector rows, for resistance to pigeonpea sterility mosaic was taken up with 591 lines (Gurdip Singh et al 1987) Seven early, six medium and thirty late maturing lines were observed resistant and free from infection While, another 16, early and 10 late lines had 10 per cent or less disease incidence

In field trials with 150'mes showing 0 100 percent infection (Gupta et al 1988) 24 were reported resistant, 34 moderately resrtant and J2 tolerant while the remainder were highly susceptible Gupta et al (1988) also screened 162 lines of pigeonpea for resistance to sterility mosaic during the rainy season of 1984-87, under artificial epiphytotic conditions Nine lines were immune while three were moderately resistant

Among 172 local and exotic accessions screened for resistance to pigeonpea sterility mosaic (Onkar Singh et al 1989) only one of local origin was completely free from infection while another, showed symptoms in only 5 per cent of the plants in Nepal Seven promising lines showed disease incidence ranging from 16 2 to 50 per cent Further of 43 advanced germplasm lines screened with artificial inoculation for resistance to pigeonpea cterility mosaic (Mishra and Prasad 1989) six belonging to the late maturity group were found free from infc ction while five lines showed less than 5 per cent infection Three lines, ICP 786 10976 and 10077 were resistant across 10 different locations tested within India (Nene et al 1989) Screening of 240 advanced breeding/ germplasm lines by hedge and leaf stapling inoculation was taken up by Chauhan eta/ (1991) and lines l24Band 125B were four d tolerant while lines 134B and 124Вг were found resistant along with other 19 lines that were free from disease

A total of 141 germplasm accessions and 725 breeding lines of pigeonpea were evaluated for resistance to sterility mosaic (Amin et al 1993) at 13 different locations in India from 1983-84 to 1989-90 The trials were artificially inoculated by either leaf-stapling or mfector-hedge method The line ICP 7035 was found resistant at 12 loc-itions while 18 other lines were observed resistant at 10 locations

The above screenmq for resistance to sterility mosaic were not against any specific strain of the disease However in view of the reported genetic plasticity of the sterility mosaic pathogen in India (Nene et al 1989 Reddy et al 1991 Reddy étal 1993) the need for screening against specific strains of the SM disease became apparent In this direction 153 lines reported resistant/tolerant (Nene et al 1981) were screened against two different strains of the disease identified by Reddy et al (1991) at ICRISAT The results indicated resistance for SM disease to be strain specific Only 37 lines were found resistant to variant 2 while 17 lines were resistant to variant 3 Fifteen lines were found resistant to both variants (Srimvas and Reddy 1995)

2 4 GENETICS OF RESISTANCE

Resistance or susceptibility of a crop to a particular pathogen is the manifestation of host parasite interaction controlled by the co evolvmo genetic systems of both the host and parasite In centers of origin and crop diversity host population conta ns a wide spectrum of protective mechanisms that ensure survival against a high diversity of pall ogemcity n the parasite This results in a hosi'parasite equilibnum and most of the host genotypes have some degree of resistance against the parasite However in new areas of crop adaptation and intensive cultivation of c particular genotype genes for virulence to overcome the narrow genetic base of the host are favored causing susceptibility in the new cultivar In the past pigeonpea cultivation in India and areas of Africa and Latin America had been confined to subsistence agriculture based on adapted landraces The development of improved varieties by hybridization and selection under expenmental conditions and cultivation in intensive production systems under irrigated conditions is a relatively recent phenomenon which has upset the delicate host/parasite equilibrium favoring the outbreak of diseases such as sterility mosaic Foi planned disease management it is essential that genetic systems operating in a given host/pathogen envronment are well understood At present studies on genetics of disease resistance in pigeonpea are limited and preliminary

Studies on inheritance of resistance to sterility mosaic disease of pigeonpea are also few and limited Singh et al (1983) studied the inheritance of resistance to sterility mosaic in 15 crosses involving five resistant and three susceptible genotypes F, F2 BC, and BC2 generations were studied Resistance was under the control of four independent non allelic genes The symbols Sv, Sv2 sv3 and sv4 were assigned to the four resistance genes Sv, and Sv2 were reported to exhibit duplicate dominant epistasis while, sv3 and sv4 exhibited duplicate recessive epistasis It was further concluded that presence of at least one dominant allele at locus 1 or 2 and homozygous recessive genes at locus 3 or 4 were essential for resistance reaction

The influence of extra nuclear factors in the control of sterility mosaic resistance was reported by Sivasubramaman et al (1983) based on observations of reciprocal differences in the study of F, and F2 of CO-3 X ICP 4782 cross

Inheritance of resistance and allelic relationships for the disease were also studied by Sharma et al (1984) in pigeonpea crosses involving susceptible tolerant (ring spot) and resistant genotypes F, and F2 generations were studied Dominance of susceptibility over resistance and tolerance was noticed in all crosses Resistant lines were however reported to differ in the expression of their resistance in crosses with tolerant genotypes Tolerance war found dominant over resistance of certain lines in few crosses, while It was recessive to resistance in other lines In crosses between resistant and susceptible lines 9 7 and 3 1 segregation ratios were observed The disease reaction in F, and segregation in F2 was explained on the basis of two genes and more than two alleles per locus Inheritance of resistance to sterility mosaic was reported to be complicated and determined by multiple allelic series

The control of resistance trait by non allelic interaction of two factors was reported by Amala Balu and Rathnaswamy (Personal Communication) They studied F, and F2 generations of four cross combinations involving two susceptible male steriles viz MS Prabhat (DT) and MS CO 5 and two resistant parents, ICPL 83024 and ICPL 83027 F, s were all susceptible indicating lominance of susceptibility over resistance while, F2s segregated in 13 susceptible 3 resistant ratio

The above inheritance studies have little significance in the wake of reports of variability in the stenlity mosaic pathogen Studies on genetics of strain-specific resistance for the disease are necessary However, such studies are lacking for pigeonpea sterility mosaic

2.5 HETEROSIS AND COMBINING ABILITY

The selection of suitable parents is important in a breeding program particularly if the aim is to improve a quantitative character such as yield The per se performance of a parent need not necessarily be a good indicator Therefore qathenng information on the nature of gene effects and their expression in terms of combining ability is necessary Further heterosis has been extensively used to realize substantial yield gains in crops like m nze sorghum bajra cotton ano castor Considerable extent of heterosis for yield and other tiaits lias been reported in many legumes (Singh 1974) including pigeonpea (Saxena et al, 1986 Saxena et al 1989 Zaverí ef al 1989) A bnef review of the relevant literature is presented hereunder

2.5.1 Heterosis

The term "Heterosis wac coined by Shull (1914) to refer to the phenomenon in which the F, obtained by crossing two gpn ’tically dissimilar individuals showed an increase or decrease in vigor over the mid-parent value The temi hetembeltiosis was proposed later (Bitzer et al 1968 Fonesca and Patterson 1968) to denote the expiession of heterosis over better parent

The potency of heterosis breeding is enormous in terms of increasing the productivity of crop plants It has already become popular in the breeding of cross pollinated crops like maize millet, onion sugarbeet and sunflower and is increasingly being utilized for enhancing be productivity of self-pollinated crops (Rai, 1979)

The discovery of heterosis in chickpea (Pal 1945) opened the way for heterosis breeding in pulses Varying degrees of heterosis with respect to yield and yield components have been observed in several pulse crops

Solomon et al (1957) were the first to report hybrid vigor in pigeonpea for grain yield A wide range of heterosis is also present for almost all characters in pigeonpea The range in percentage of mid and better parent and standard hotero is for different characters is presented in Table 3 The expression of heterosis is most evident tor plant height branch number pod number plant spread and cluster number (Veeraswamy et al 1973) A mean heterosis of 80 per cent for number of pods per plant was reported

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(Shrivastava et al., 1976) over the better parental values. Medium x mevum and low x medium crosses were generally observed to result in high heterotic performance over the better parent.

The magnitude of heterosis for yield and related characters between crosses involving different maturity groups was investigated by Reddy et al. (1979). The study revealed negative heterosis over better parent for plant height, days to flower, days to maturity and seed weight while, heterosis for pod number and seed yield over better parent were generally positive. Yield as well as heterosis were found maximum in early x late and medium x late crosses involving diverse plant types. Hybrids based on mid-late parents were also reported to give higher hybrid vigor as compared to those with early parents (Patel,1988).

A considerable degree of heterosis was observed among a set of 63 hybrids derived through line x tester mating between three genetic male sterile lines and 21 short duration pollen parents, in respect of seed yield and componeni characters (Rao, 1989). The hybrids based on mid-late females recorded greater hybrid vigor compared to those based on early females. Significant standard heterosis with regards to yield, over C 11 parent for all hybrids studied, in a 5 x 5 diallel was reported by Cheralu et al. (1989).

Favorable heterosis for developmental traits such as plant height and number of days to 50 per cent flowering to complete maturity was also noticed for six early Cajanus cajan hybrids studied at Varnasi, India during 1987-88 (Singh et al.. 1989). Most of these hybrids were also heterotic for number of pods per plant. Positive heterosis for plant height, seeds per pod and seed yield was also recorded in 15 medium- duration hybrids, obtained from crosses between male-sterile ICP 3783 and 15 advanced breeding lines.

A high expression of heterosis for seed yield was recorded for a set of 45 hybrids (Rana, 1990) derived through line x tester mating between three genetic male sterile lines and 15 short duration pollen parents. The heterosis for seed yield was found associated with greater amount of heterosis for component characters like number of pods per plant, branches per plant and per day production.

The study of Patel (1990) involving 45 hybrids obtained from three male sterile lines and 15 medium duration pollinators, crossed in a line x tester fashion also revealed a profound degree of useful and significant heterosis for days to flowering, days to maturity, branches per plant, seeds per pod, seed yield per plant and per day production.

High heterosis for seed yield per plant due to high heterosis for pods per plant, plant height and branches per plant was noticed for 60 hybrids grown during 1985-86 (Patel et al., 1991 ). Hybrids with two early parents were superior for early maturity but not for yield, while high heterotic hybrids had at least one medium maturing parent.

Patel and Patel (1992) indicated highest heterotic response for number of pods per plant, for 30 hybrids obtained from six diverse pigeonpea lines, crossed with five testers. It was followed by seed yield per plant. Mehetre ef al. (1992) noticed significant and positive heterosii to an extent of 60 per cent tor days to 50 per cent flowering and 56.9 per cent for days to maturity in their study of 9 x 9 diallel crosses of pigeonpea.

Significant heterosis of few determinate and indeterminate hybrids over checks (ICPH 8, UPAS 120 and Manak), for seed yield and yield component characters was recorded by Bajpai et al. (1994). Desirable relative heterosis for seed yield in 38 hybrids out of a total of 60 cross combinations was also reported by Sinha et al. (1994). Heterosis was also noticed for pods/cluster, pods per plant and 100-seed weight while, poor or negative heterosis was recorded for seeds per pod. Malik et al. (1995) reported low heterosis in cross combinations of pigeonpea involving less divergent parents. Crosses involving divergent parents also exhibited low or no heterosis when majority of the dominant alleles were present in one parent and majority of the recessive alleles in the other parent, coupled with the absence of overdominance.

2.5.2 Combining ability and gone action

The concepts of general and specific combining abilities were coined by Sprague and Tatum (1942). General combining ability (GCA) was defined as the average performance of a line in hybrid combinations, while specific combining ability (SCA) referred to those crosses, wherein certain hybrid combinations did relatively better or worse than was expected, on the basis of average performance of the lines involved.

Griffing (1956) pointed out the usefulness of information on the relative magnitude of additive and non-additive gene effects in designing an efficient breeding program. The information could be obtained through the study of combining ability, as variance due to GCA involved mostly additive gene action while that, due to SCA involved dominance and epistatic components ot genetic variances. The need to study combining ability in sell-pollinated crops was stressed by Allard (1960).

Sidhu and Sandhu (1981) and Reddy ef al. (1981) had summarized the results of studies on combining ability and gene action in pigeonpea. Yield in general appeared to be additively inherited (Green et al., 1979). Pre-ponderance ot additive gene action was also observed tor majority of the traits (Sharma et al., 1973b; Venkateswarlu and Singh, 1982; Lakhan et al., 1986). The nature of gene action for various traits in pigeonpea as reported by different workers is summarized in Table 4. The estimates of gca effects of individual parental lines recorded a close agreement with ranking of the lines for such effects and ranking based on parental performance per se (Sharma et al., 1973b; Venkateswarlu and Singh, 1982). The best cross between two parents was reported to be the one. chosen on the basis of low gca for flowering time and high gca for other traits (Dahiya and Brar, 1977). The gca effects for most characters were generally negative, for early and medium parents and positive for late groups (Reddy et al.. 1979). Specific medium x late and early x late cross combinations were reported more likely to yield recombinants of economic worth.

2.6 GENETIC VARIABILITY

The phenotypic expression of quantitative characters is a combination of the genotype, environment and their interaction. Further, progress of selection in a population is conditioned by the nature and magnitude of variation. A wide range of genetic variability is reported for virtually all important agronomic characters (Sharma and Green, 1977) in pigeonpea.

Bashiruddin and Sreeramulu ( 1981 ) reported high genotypic coefficient of variation (GCV) for 100- seed weight, cluster number and pod number, and low GCV for seed number. Highest estimates of GCV were also reported for pods per plant and seed yield per plant (Jag Shoran, 1985; Natarajan et al., 1990; Holker et al., 1991 ; Patel and Patel. 1992). High variability for pods per plant and low variability for seeds per pod was also reported by Sidhu et al. (1985). Moderate to high GCV values were reported for number of primary branches and secondary branches by Balyan and Sudhakar (1985). High GCV for number of

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branches per plant was also reported by Patii et al. (1989). Further, high GCV for days to maturity and plant height was reported by Saxena and Kataria (1993).

The traits, days to 50 per cent flowering and days to maturity were found to be less influenced by environment, in comparison to seed yield, seed size, seeds per pod, pods per plant and plant height (Sidhu et al., 1985). Natarajan et ai (1990) reported minimum difference between phenotypic and genotypic coefficient of variations for 100-seed weight while, branch number and seed number exhibited wider gap between PCV and GCV.

2.7 HERITABILITY AND GENETIC ADVANCE

Observed variability is a combined measure of genetic and environmental causes. Genetic variability alone is heritable. However, heritability has to be considered in conjunction with genetic advance (Natarajan et al., 1990) to have an idea about the expected genetic gain in the next generation.

The maximum and minimum values of broad sense heritability for different traits of pigeonpea are presented in Table 5. High heritability estimates were reported for pod number, and cluster number (Suresh Kumar and Reddy, 1982; Premsagar and Jatarsa, 1983; Natarajan et al., 1990); seed yield (Premsagar and Jatarsa, 1983; Natarajan et at.. 1990); days to flowering (Singh et al.. 1979; Gupta et al., 1980; Konwar and Hazarika, 1988; Holker et al. 1991); days to maturity (Konwar and Hazariaka, 1988: Holkar et al., 1991); 100-seed weight, plant height and number of secondary branches (Konwar and Hazarika, 1988). Low heritability estimates wem recorded for pods per cluster, primary branches, pods per plant, seed per pod (Konwar and Hazarika, 1988) and 100-seed weight (Gupta et al, 1980). High genetic advance was reported for cluster number and seed yield (Bashiruddin and Sreeramulu, 1981; Premsagar and Jatarsa, 1983; Natarajan et al, 1990)

Number of leaves per plant and seeds per plant had exhibited high heritability in broad sense and high genetic advance as per cent ot mean (Kumar and Haque, 1973). Days to flowering and days to maturity (Konwar and Hazarika, 1988; Holker et al, 1991); plant height (Konwar and Hazarika, 1988) and pods per plant (Holker et al, 1991) were also reported to exhibit high heritability and genetic advance in pigeonpea.

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2.8 CORRELATIONS AND PATH-COEFFICIENTS

Yield is a complex character governed by several contributing traits. Hence, study of associations of component characters with yield, would aid in planning of efficient breeding programs. A brief review of the relevant literature is presented hereunder.

Grain yield in pigeonpea was reported to be positively correlated with days to flowering (Veeraswamy et ai, 1973: Patii et al., 1989): plant height (Sidhu étal., 1985: Patii étal.. 1989: Natarajan et al., 1990; Patel and Patel. 1992): total number of branches (Beohar and Nigam, 1972; Joshi, 1973; Veeraswamy et al., 1973); primary branches (Wakankar and Yadav, 1975): secondary branches (Sharma et al., 1971; Singh and Malhotra 1973: Wakankar and Yadav, 1975), pod bearing length (Sharma et al., 1971); pods per plant (Sidhu et al. 1935: Patii et al., 1989; Natarajan et ai, 1990; Patel and Patel, 1992); seeds per pod (Sidhu et al., 1985 Patii et al., 1989) and with 100-seed weight (Patii et al.. 1989). However, non-significant association between days to flowering and days to maturity with seed yield was also reported by several workers (Pankaj Reddy et ai. 1975: Dahiya et ai, 1978; Sidhu et ai, 1985) while, Patii et ai (1989) reported significantly negative association of seed yield with days to maturity. The negative association of plant height with seed yield (Dahiya et ai. 1978) and pods per plant with seed yield (Beohar and Nigam, 1972) were also reported.

Positive associations of plant height with branch number (Natarajan et ai, 1990); seeds per pod (Sidhu et ai, 1985); days to flowering (Sidhu er ai, 1985; Patel and Patel, 1992); pods per plant (Sidhu et ai, 1985; Patel and Patel, 1992): 100-seed weight (Natarajan et ai, 1990); and primary branches per plant (Patel and Patel, 1992) has been reported Branch number was also reported to be positively and significantly associated with seed number and 100-seed weight (Natarajan ef ai, 1990). Further, primary branches per plant was reported to exhibit significant positive association with days to flowering and days to maturity (Patel and Patel, 1992) while, days to flowering was positively and significantly associated with days to maturity (Patel and Patel. 1992) . Seeds per pod was found positively associated with 100-seed weight (Patel and Patel, 1992). Negative association was observed for days to flowering and days to maturity with seeds per pod (Sidhu ef ai, 1985).

The technique of path analysis was outlined by Wright (1921) for partitioning the observed correlation into direct and indirent effects It was applied in plant breeding for the first time by Dewey and Lu (1959).

Path analysis in pigeonpea revealed the highest direct effect of pods per plant on seed yield (Dumbre and Deshmukh, 1985; Sidhu et al.. 1985; Natarajan et al., 1990). However, seeds per pod (Jag Shoran.1982; Singh et al.. 1982: Patii el al., 1989) and days to maturity (Patel and Patel,1992) were reported to exert high direct effect on seed yield in other studies. On contrary, Baniwal and Jastra (1985) reported high negative direct effect of seeds per pod on seed yield. Days to flowering was reported to exert indirect effect on seed yield via plant height, pod' per plant (Sidhu et al.. 1985; Patel and Patel, 1992) and also via seeds per pod and 100-seed weight (Patel and Patel, 1992). Plant height and pods per plant were found to be the most important contributors to yield in pigeonpea (Sidhu et al. 1985; Natarajan et al, 1990) while, number of seeds per pod. days to flower. '00-seed weight and number of branches per plant were also reported important (Patii et al. 1989) in pigeonpea improvement programs.

CHAPTER III MATERIALS AND METHODS

The present investigations were carried out at ICRISAT Asia Center (IAC), Patancheru, India during 1993-1996.

3.1 EXPERIMENTAL MATERIAL

3.1.1 Pathogen

Three different isolates of pigeonpea sterility mosaic pathogen viz., isolate 1,2 and 3, representing the variants 2, 3 and 1, respectively, of those, identified by Reddy et al. (1993), were involved in the present investigations. The identity of these isolates was established by their reaction on few pigeonpea differentials presented below

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R-Resistant (No apparent symptoms); S-Susceptible (Mosaic symptoms); RS-Ring Spot; NT-Not tested

The isolates involved in the study, were obtained from SM infected pigeonpea fields of local cultivars, located at different places within Andhra Pradesh. Isolate 1 was collected from the infected pigeonpea fields of Bibinagar Mandai of Nalgonda district, during January 1993 while, isolates 2 and 3 were obtained from the SM infected fields of Narsapur Mandai, Medak district, during November 1994 and Ghanpur village of Ramachandrapuram Mandai, Medak district, during September 1995, respectively. The inoculum carrying sufficient number of mites (7-10 per leaf, on average) was brought in moistened muslin cloth bags and used for inoculation of seedlings of pigeonpea differentials at primary leaf stage, including the susceptible, ICP 8863, by leaf-stapling technique (Nene and Reddy, 1976a). Observations were recorded on both disease incidence and symptom type (no apparent symptoms, ring spot and mosaic symptoms), two months after inoculation.

Multiplication of the isolates was taken up after confirmation, in isolation, on the susceptible cultivar, ICP 8863, grown in pots, at different locations to avoid cross-contamination. Isolate 1 was multiplied in the residential areas of Hyderabad, devoid of any pigeonpea within a radius of 5Kms while, isolate 2 was multiplied in the SM and wilt screening nurseries of ICRISAT Asia Center, Patancheru, Andhra Pradesh. The inoculum of isolate 3 was however, multiplied in a mite-proof nethouse at ICRISAT Asia Center, Patancheru, Andhra Pradesh. The inoculum of the three different isolates, thus multiplied was used for subsequent screening experiments.

3.1.2 Host

Parents for the investigation on inheritance of resistance to sterility mosaic disease of pigeonpea were selected from a preliminary screening experiment. Lines, earlier reported resistant/ tolerant (Nene ef al., 1981) were evaluated for their reaction against different isolates of the sterility mosaic pathogen. The selected parents are presented in Table 6 Details regarding their origin and other salient features are presented in Table 7. Crosses were made with the susceptible parents and part of the F, was advanced to F2 generation. Backcrosses were also made simultaneously. The parents. F, and segregating generations were then screened against the isolates to determine the mode of inheritance of resistance. The resistant and susceptible parents selected for isolate 2 were also intercrossed among themselves to obtain information on their allelic relationships.

The selected pigeonpea lines were further crossed in a line x tester fashion. The lines, ICP 2376, ICP 7035, ICP 7349, ICP 7994, ICP 8006, ICP 8136, ICP 8850, ICP 8863, ICP 11251, BDN 1 and LRG 30 were crossed with the male steriles, ICP MS288 and ICP MS3783 and the resulting 22 F, hybrids along with the parents, including standard checks constituted the material for study on heterosis, combining ability and nature of gene action.

Table 6 Parent's selected tor studies on inheritance ot resistance to different isolates of the sterility mosaic pathogen

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3.2 METHODS

3.2.1 Screening for resistance to Isolates of sterility mosaic pathogen

A set of 152 lines, earlier reported resistant/tolerant (Nene et al., 1981) were screened against isolate 1 of the sterility mosaic pathogen. The lines were screened for their reaction, during May-July 1993. Screening was carried out using the infector-hedge technique (Nene and Reddy, 1976b). The infector- hedge was established by growing the susceptible cultivar, ICP 8863 on the upwind border of the field (Plate 2 a)A). Ten days old seedlings of the hedge were inoculated with the isolate 1 of the SM pathogen, by leaf-stapling (Nene and Reddy, 1976a) and spreading of diseased twigs infested with mites among the seedlings. The pathogen and mites that multiplied on the hedge plants served as source of inoculum. Disease spread occurred through wind onto the test materials during the screening period. For screening, the pots sown with the test material were placed beside the infector-hedge (Plate 2 a)B)

The screening for isolate 1 was done in two replications. Plastic pots, 15 cm in diameter, were filled with affiso! (60% sand, 33% clay, 7% silt) and ten seeds were sown in each pot. These pots were then placed beside the infector-hedge Susceptible checks. BDN 1, LRG 30 and ICP 8863 were planted at frequent intervals for indication on disease spread Observations on symptom type and severity, were recorded at 75 days after sowing (DAS), when the susceptible controls had exhibited 100 per cent severe mosaic symptoms on each individual plant, in each entry and replication, individuals with no apparent symptoms were classified as resistant (Plate 2 b)A) while, those with ring spot (green islands surrounded by a chlorotic halo) and mild mosaic (few mosaic patches) symptoms were classified as tolerant (Plates 2 b)B and C). Those exhibiting severe mosaic symptoms were classified as susceptible (Plate 2 b)D). Lines with less than 10 per cent disease, over replications, were classified as resistant. While, lines with either ring spot or mild mosaic symptoms and less than 10 per cent severe mosaic symptoms were classified as tolerant (ring spot) or tolerant (mild mosaic), respectively. Lines exhibiting more than 10 per cent severe mosaic symptoms were classified as susceptible

Pot-screening of the lines using infector-hedge technique was also adopted for isolate 2. A set of 410 lines, earlier reported resistant/tolerant for the disease (Nene et al., 1981) including the 152 lines

Plate 2

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screened against isolate 1 were sown in a single replication besidesthe intector-hedge. Susceptible checks, BDN 1, LRG 30 and ICP 8863 were placed at trequent intervals for indication on disease spread. Plastic pots, 15 cm in diameter, filled with alfisols group (60% sand, 33% clay, 7% silt) were used. Ten seeds were sown In each pot. Observations on the symptom type and severity were recorded tor each plant of each entry, in each replication, at 75 DAS. The lines were classified as resistant, tolerant (ring spot), tolerant (mild mosaic) and susceptible, similar to that of isolate 1.

3.2.2 Selection of parents for Inheritance studies

Resistant and tolerant parents were selected from the preliminary screening experiment, for study on inheritance of resistance. Lines of medium to late maturity duration exhibiting uniform reaction or symptom type across the replications were selected as parents,

3.2.3 Hybridization

Crosses were made between the selected parents during Kharlf 1993. Reciprocal crosses were avoided. The parents were sown in four sets at intervals of 15 days in 30 cm pots and placed beside the infector-hedge. The susceptible parents were however, raised under disease-free conditions, as the disease would have prevented their flowering, essential for crossing. The confirmed resistant and tolerant plants, alone, were used for crossing with susceptible parent, Further, hybridization was carried out on true-to type, vigorous and healthy plants raised in 30 cm pots (Plate ЗА).The hybridization was restricted to early phase of flowering, because of higher success rates (Ramanatha Rao, 1988). All flowers on the female parents were removed, at the onset of flowering, for one-two days to stimulate profuse flowering.

For crossing, upto ten tightly closed buds, approximately two-thirds the size of mature buds were selected on each branch of the female plant. Smaller, mature, open buds and flowers were removed to prevent competition for photosynthates within the inflorescence. These buds were emasculated (Plate 3B) to avoid setting. Standard hybridization technique detailed by Pathak (1970) and Sharma and Green (1980) was adopted. Urge, mature, unopened buds with abundant pollen were collected from the male plants and bulked for each male parent. The staminal column of the pollen bud was extracted and used for pollinating the stigma of female (Plate 3C). Each female plant was pollinated by several male parents. Different

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colored threads were tied to each flower (Plate 3C) to facilitate identification of crosses at the time of harvest. The pollinated buds were not bagged as pod setting was greatly reduced under bagging (Sharma and Green, 1980). However, hybridization was carried out under bee-proof nylon cages (Plate 3D) to prevent any chance of contamination by natural out-crossing.

3.2.4 Generation advancement

Off-season advancement of the F,’s was taken up during December 1993, under greenhouse conditions to facilitate the rapid advancement of generations. Flower initiation, flower color, seed size and other contrasting characters among the parents (Table 7) were used as makers to check the trueness of F, plants. Only true F,’s were used for backcrossing and advancement to F2 generation. The growth of F, plants was hastened by providing extra light (14 hr) while, flowering was induced by providing short days of 8 hrs light and 16 hrs dark in a black out facility (Plate 4 a)A) at lAC's Greenhouse and Controlled Environment Facility. Backcrossing of the F,’s with their respective parents, maintained as ratoon (Plate 4 a)B) was Initiated under greenhouse conditions with the commencement of flowering in the F,’s, raised in the black-out facility. The F,'s were also advanced to F2 generation, during Kharlf 1994, by setting in bee-proof nylon cages.

3.2.5 Screening of parents, F, and segregating generations

The parents, F, and segregating generations were screened for their reaction to sterility mosaic disease during 1995-1996. Seedlings were raised in 15 cm pots with ten seedlings per pot.

Screening against isolate 1 of the sterility mosaic pathogen was taken up in a mite-proof net-house (Plate 4 b) during May-July 1995 using the leaf-stapling technique (Nene and Reddy, 1976a). Diseased leaflets carrying sufficient numbers of the vector, Aceria cajani were stapled to the primary leaves of test seedlings. One diseased leaflet per primary leaf was generally used. The diseased leaflet was folded on the primary leaf In such a way that its lower surface came into contact with the pnmary leaf of the test seedling (Plate 4 c)A). It was then stapled with a small paper stapler. Alternatively, two diseased leaflets

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were used, If they were too small (Plate 4 c)B). The leaflets were placed in such a way that the tower surface of one of the leaves came in contact with the lower surface of the primary leaf while, the tower surface of the other was in contact with the upper surface of the primary leaf (Plate 4 c)B). The primary leaf and the two diseased leaflets were then stapled together.

For isolate 2, the parents, F, and segregating generations were screened using the infector-hedge technique (Nene and Reddy, 1976b) in an isolated field during May-July 1995 while, for isolate 3, screening was taken up in a mite-proof net-house using the leaf-stapling technique, during Dec. 1995-Feb. 1996.

The susceptible check, ICP 8863 was included in all sets, at frequent intervals for an indication of disease spread. Observations on disease reaction were recorded at 75 DAS. The plants were classified as resistant (no apparent symptoms), tolerant (ring spot symptoms) or susceptible (severe mosaic symptoms) for isolate 1 and as resistant (no apparent symptoms) and susceptible (severe mosaic symptoms) for isolate 2 and isolate 3.

3.2.6 Evaluation tor heterosis and combining ability

The investigation consisted of a line x tester trial with eleven pollen parents (ICP 2376, ICP 7035, ICP 7349, ICP 7994, ICP 8006, ICP 8136. ICP 8850, ICP 8863, ICP 11251, BDN 1, and LRG 30) and two male steriles (ICP MS288 and ICP MS3783). The resultant 22 hybrids were evaluated along with the parents (including checks) in randomized block design of three replications. Each plot consisted of a single row of four meters length. A spacing of 75 x 20 cm was adopted. The experiments were planted on a medium deep vertisol at ICRISAT Asia center, Patancheru on 24,h of June 1994 and all recommended package of practices were adopted to raise a successful crop.

Observation

Apart from days to 50 per cent flowering and days to maturity, observations for all other traits were recorded on ten randomly selected plants, from the center of each plot. For days to flowering and maturity, observations were however, recorded from the entire plot. The hybrid plants were identified at flowering and pod formatton stages, by comparing various plant characteristics, such as flower, pod and stem pigmentation with that of parents. Any off-type noticed was promptly rouged out.

Days to 50 per cent flowering :

Number of days from sowing to the day when 50 per cent of the plants in the plot had flowered.

Days to maturity :

Number of days taken from sowing to the day when 75 per cent of the pods in the plot had turned brown and matured.

Plant height (cm) :

Height of stretched plant from ground level to its tip at harvest.

Number of primary branches :

Number of branches arising form the main-stem recorded at harvest.

Number of secondary branches :

Total number of branches arising from primary branches recorded at harvest.

Number of pods per plant :

Total number of mature pods per plant observed at harvest.

Number of seeds per pod :

The average of observations on ten fully developed, mature, undamaged pods taken at random from each selected plant.

Plant seed yield (g) :

The average seed weight of ten randomly selected plants measured to the nearest grams.

100 Seed weight (g) :

The weight of randomly collected one hundred, clean, whole, dry seeds.

3.3 STATISTICAL ANALYSIS

3.3.1 Inheritance studies

Observations on disease symptom type and severity were recorded on the parents, F, and segregating generations. The plants were classified as resistant, tolerant and susceptible. The Chi-square

method (Snedecor and Cochran, 1967) was adopted to test the goodness of fit for the phenotypic ratios.

3.3.2 Analysis ot variance

The data for each trait was analyzed separately. Randomized complete block design method, suggested by Panse and Sukhatme (1978) was adopted. The treatment sum of squares in the ANOVAwas further partitioned as per the procedure outlined by Singh and Chaudhary (1985).

3.3.3 Heterosis

The performance of F, hybrid over the mid-parent, best parent and checks was expressed as per cent for each cross. It was calculated using the formula suggested by Liang et al. (1972). The significance was tested using t-test suggested by Snedecor and Cochran (1967) and Paschal and Wilcox (1975).

3.3.4 Combining Ability

The analysis of combining ability was carried out based on the methods suggested by Comstock and Robinson (1952) and Kempthome (1957). The estimates of heritably and genetic advance were also obtained adopting the procedures outlined by Burton and Devane (1953) and Johnson et al (1955), respectively. The genotypic and phenotypic coefficients of variation were computed, following the methodology outlined by Burton (1952).

3.3.5 Character associations and Path analysis

Correlations for various traits studied, were computed using the statistical procedures outlined by Singh and Chaudhary (1985). The direct and indirect effects for yield, were estimated with the various yield components as independent variables. The procedures suggested by Wright (1921) and Dewey and Lu (1959) were adopted.

CHAPTER IV RESULTS

Results ot the present investigations on "Genetics ot resistance to sterility mosaic disease in pigeonpea" are presented hereunder

4.1 SCREENING FOR RESISTANCE TO DIFFERENT ISOLATES

A set ol 152 lines were screened for resistance to isolate 1 ot the pigeonpea sterility mosaic pathogen. Disease incidence varied from 0-100 per cent in different lines The susceptible checks (BDN 1, LRG 30 and ICP 8863). showed 90-100 per cent disease, indicating a good spread of the disease. Among the 152 lines screened. 37 lines were resistant (less than 10 per cent disease incidence), while 83 lines exhibited tolerance (29 ring spot and 54 mild mosaic) and the rest (32 lines) were susceptible. The list ot resistant and tolerant lines is given in Table 8

Screening tor resistance to isolate 2 of the pigeonpea sterility mosaic pathogen was carried out with a set of 410 lines including 152 lines screened against isolate 1. The susceptible controls (BDN 1. LRG 30 and ICP 8863) placed at frequent intervals exhibited 100 per cent disease indicating good disease spread. Among the lines tested. 161 were resistant while 53 were found tolerant against the isolate. These lines are presented in Table 9

Among 152 lines screened for resistance to both isolates. ICP 2630, ICP 3782, ICP 3783, ICP 4725, ICP 7035, ICP 7239. ICP 7281. ICP 7349. ICP 7403, ICP 7867, ICP 8116. ICP 8117, ICP 8850, ICP 8853, ICP 8861 and ICP 11278 exhibited resistance to both isolates. Similarly, the line ICP 11245 recorded ring spot form ot tolerance to both isolates, while the lines ICP 999, ICP 7201. ICP 7873. ICP 8125, ICP 8266, ICP 8857, ICP 11249 and ICP 11283 showed mild mosaic form of tolerance to both isolates.

4.2 GENETICS OF RESISTANCE FOR ISOLATES OF PIGEONPEA STERILITY MOSAIC PATHOGEN

The results on inheritance of resistance to three different isolates of the pigeonpea sterility mosaic

Pigeonpea lines resistantAolerant to isolate 1 of sterility mosaic pathogen

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pathogen are presented in Tables 10-17

4.2.1 Genetics of resistance for Isolate 1

The F„ F? and backcross generations of five resistant x susceptible and one tolerant x susceptible cross combinations were studied to determine the inheritance of resistance/tolerance for isolate 1. The susceptible control, planted along with test materials, exhibited 100 per cent infection, indicating good disease spread. The parents, ICP 7035, ICP 7349, ICP 8006. ICP 8136 and ICP 8850 showed 100 percent resistance, with no apparent symptoms, while ICP MS3783 showed ring spot form of tolerance. ICP 8863 recorded severe mosaic symptoms (Table 10) The F,'s were susceptible for all resistant x susceptible and tolerant x susceptible cross combinations studied

A segregation of 7 resistant 9 susceptible was observed in the F? generation of crosses involving ICP 7035, ICP 7349 and ICP 8006 resistant parents with the susceptible, ICP 8863 (Table 11). However, crosses with resistant parents. ICP 8136 and ICP 8850 showed 1 resistant : 3 susceptible segregation ratio. The backcrosses corroborated the segregation pattern of F? generation (Table 11). The backcross of ICP 7035 X ICP 8863. ICP 7349 X ICP 8863 and ICP 8006 X ICP 8863 F.'s with the respective resistant parents, segregated in a ratio of 3 resistant 1 susceptible While, backcross with the susceptible parent, did not segregate and the entire progeny was susceptible The backcross of ICP 8850 X ICP 8863 F, with the resistant parent segregated in a segregation ratio of 1 resistant. 1 susceptible while, backcross with the susceptible parent did not segregate and the entire progeny was susceptible The backcross of ICP 8136 X ICP 8863 F, with the resistant parent, also segregated in a ratio of 1 resistant 1 susceptible while, the entire backcross progeny ot the F with the susceptible parent was susceptible.

The F, of cross. ICP MS3783 X ICP 8863 was susceptible (Table 10). Further, the F2 plants exhibited a segregation ratio of 7 tolerant 9 susceptible (Table 11). The segregation pattern of F2 generation was supported by the segregation ratios obseived in BC, and BC2 generations. The backcross of F, with the tolerant parent segregated m the ratio 3 tolerant 1 susceptible, while backcross of the F, with the susceptible parent resulted m susceptible backcross progeny

4.2.2 Genetics of resistance tor Isolate 2

The cross combinations o. three res,Stan, paients. ICP 7035. It- 7349 and tCP 8850 with Six

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susceptible parents, ICP 2376, ICP 7994, ICP 11251, BDN 1, LRG 30 and ICP 8863 were studied to determine the genetics of resistance for isolate 2 of pigeonpea sterility mosaic pathogen. The susceptible controls, planted at frequent intervals along with the test materials exhibited 100 per cent infection, indicating good disease spread. The parents, ICP 7035, ICP 7349 and ICP 8850 did not exhibit any apparent symptoms and were 100 per cent resistant, while ICP 2376, ICP 7994, ICP 11251, BDN 1, LRG 30 and ICP 8863 exhibited severe mosaic symptoms (Table 12).

The disease reaction of F, hybrids of resistant x susceptible cross combinations is presented in Table 13. F,'s of crosses involving ICP 7035 and ICP 7349 resistant parents were all resistant, while F, of crosses involving ICP 8850 were all susceptible. In the F2 generation (Table 14), the crosses of resistant parents, ICP 7035 and ICP 7349 with ICP 2376, BDN 1 and ICP 8863 segregated in the ratio 3 resistant : 1 susceptible, while crosses with the susceptible parents ICP 7994, ICP 11251 and LRG 30 segregated in 9 resistant : 7 susceptible ratio. However, in the F2 generation of the crosses involving the resistant parent, ICP 8850 and the susceptibles, ICP 2376, BDN 1 and ICP 8863, 1 resistant : 3 susceptible ratio was observed while, in combination with the susceptible parents, ICP 7994, ICP 11251 and LRG 30, 3 resistant : 13 susceptible ratio was noticed.

The F,'s of all resistant x resistant crosses were resistant, while F,'s of all susceptible x susceptible crosses were susceptible (Table 15). Further, no segregation was observed in the F2 generation of either resistant x resistant or susceptible x susceptible crosses (Table 16)

4.2.3 Genetics of resistance for isolate 3

The cross combinations of two resistant parents, ICP 7035 and ICP 2376 with the susceptible, ICP 8863 was studied to elucidate the genetics of resistance for isolate 3 (Table 17). The parents, ICP 7035 and ICP 2376 did not exhibit any symptoms and were resistant. On the other hand, ICP 8863 recorded severe mosaic symptoms and was susceptible. The F,‘s of the resistant x susceptible crosses were all susceptible. In the F2 generation, a segregation ratio ol 7 resistant : 9 susceptible was observed for the cross, ICP 7035 X ICP 8863. However, the cross, ICP 2376 X ICP 8863 segregated in 1 resistant : 3 susceptible ratio.

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